1. Field of the Invention
This invention relates to a block for securing a wire and, more particularly, to such a block for securing a wire having polytetrafluoroethylene insulation. The invention also relates to an electrical switching device having auxiliary wiring with polytetrafluoroethylene insulation and a block for securing such wiring.
2. Background of Information
Electrical switching devices include, for example, circuit switching devices and circuit breakers. Circuit breakers are generally used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload fault or a relatively high level short circuit condition. Molded case circuit breakers, for example, include at least one pair of separable contacts which may be operated either manually by way of a handle disposed on the outside of the case or automatically in response to an overcurrent condition. In the automatic mode of operation, an electronic trip unit, for example, controls an operating mechanism which opens the separable contacts. In the manual mode of operation, the handle, for example, cooperates with the operating mechanism in order to open the separable contacts. Electrical switching devices have a line terminal for connection to a power source and a load terminal for connection to a load, such as a motor. The separable contacts of the electrical switching device are internally connected to the line and load terminals.
Electrical switching devices may also have one or more auxiliary connections. In a circuit breaker, for example, such auxiliary connections may be utilized as inputs to provide an external signal for tripping the circuit breaker. Input auxiliary connections may include, for example, connections to a shunt trip mechanism or an undervoltage trip relay which drive an internal solenoid that trips the circuit breaker. Furthermore, other such auxiliary connections may be utilized as outputs to externally indicate the status of the circuit breaker. Output auxiliary connections may include, for example, connections from an internal auxiliary switch to a bell alarm and/or other external circuits for enunciating and/or monitoring the open/closed/tripped status of the circuit breaker. The shunt trip mechanism, the undervoltage trip relay, the internal solenoid, the internal auxiliary switch, the bell alarm, the other external circuits, and other auxiliary devices are generally referred to as attachments.
Some circuit breakers include an external auxiliary terminal block which is utilized for terminating external wires between an external attachment and the terminal block and, also, for terminating internal wires between an internal attachment and the terminal block. Other circuit breakers (e.g., for residential and light industrial and commercial use) do not include an external auxiliary terminal block. In these circuit breakers, auxiliary wires are routed from an internal attachment inside the circuit breaker and are directly connected to an external attachment outside the circuit breaker.
Electrical switching devices generally have limited internal space for the separable contacts, the operating mechanism, the electronic trip unit, and the attachment. In some applications, this results in a relatively high internal operating temperature (e.g., 85.degree. C.). In order to function properly under such internal temperature, electrical switching devices generally utilize auxiliary wires having polytetrafluoroethylene insulation (e.g., "TEFLON", etc.).
As understood by those skilled in the art, polytetrafluoroethylene is sufficiently inert that nothing will bond to it. Furthermore, it is suitably slippery that conventional clamping techniques (e.g., pressure clamps, wire ties, tie-wraps, etc.) cannot adequately hold the polytetrafluoroethylene insulation of the auxiliary wire. Known techniques for clamping such wire involve wrapping the wire around a shaft or providing a large number of bends in order to prevent the wire from being pulled in a straight path. In the limited internal space of an electrical switching device, such techniques subject the wire to stress and, hence, there is room for improvement.
Underwriters Laboratory (UL) provides a variety of tests for the certification of electrical equipment. In a pull-out test, according to UL, each auxiliary wire of an electrical switching device must support a 20 pound weight for one minute. During such test, the auxiliary wire must be disconnected from the internal attachment (e.g., the terminals of an auxiliary switch).
There is a need, therefore, for an improved mechanism for securing a wire having polytetrafluoroethylene insulation.
There is also a need for an improved mechanism for securing an auxiliary wire of an electrical switching device.
There is a more particular need for such a mechanism which secures an auxiliary wire having polytetrafluoroethylene insulation.
There is another more particular need for such a mechanism which secures an auxiliary wire in the limited internal space of the electrical switching device.